The disclosed embodiments of the present invention relate to generating a regulated output voltage, and more particularly, to methods and control circuits for controlling a buck-boost converting circuit to generate a regulated output voltage under a reduced average inductor current.
A conventional regulator is employed for providing a regulated output voltage according to an input voltage. FIG. 1 is a diagram illustrating a conventional buck-boost converting circuit 100. The buck-boost converting circuit 100 is for generating a regulated output voltage V_OUT according to an input voltage V_IN, and includes an input capacitor 102, an inductor 104, an output capacitor 106, and a plurality of switches SW_A, SW_B, SW_C, and SW_D, where the voltage difference between one terminal of the input capacitor 102 and the ground terminal GND is the input voltage V_IN, and the voltage difference between one terminal of the output capacitor 106 and the ground terminal GND is the regulated output voltage V_OUT. The conventional regulator control switches on (turns on) the switches SW_A and SW_C together and switches on (turns on) the switches SW_B and SW_D together, alternately. That is to say, switches SW_A and SW_C are switched on when switches SW_B and SW_D are switched off, and switches SW_A and SW_C are switched off when switches SW_B and SW_D are switched on. In this way, the regulated output voltage V_OUT and the input voltage VIN would have the following transfer function: V_OUT/V_IN=D/(1−D), where D represents the turn-on duty of the switches SW_A and SW_C.
FIG. 2 is a waveform diagram illustrating the inductor current Iind of the inductor 104 and the current ID of the switch SW_D. As the switch SW_D is only turned on according to the turn-on duty (1−D), the average inductor current should be high enough to supply the desired output current to an output load (not shown). As can be seen from FIG. 2, the peak inductor current is very high, which is undesirable as the switches, the direct-current resistance (DCR) of the inductor, and/or other parasitic resistance in the current path would consume much power. Specifically, the relation between the average inductor current IAVG, L of the inductor 104 and the average current IAVG, SW-D of the switch SW_D can be expressed using the following equation: IAVG, SW-D=IAVG, L*(1−D). In a case where V_OUT/V_IN=3.3V/3.3V, and the required output current is 300 mA, the turn-on duty D is about 50%, and the average inductor current IAVG, L will be about 600 mA, which is twice as large as the average output current 300 mA. However, the large average inductor current will result in large conversion efficiency loss.
Thus, there is a need for an innovative regulator control design to make a buck-boost converting circuit generate a regulated output voltage under a reduced average inductor current.